Robert Andrews Millikan

Robert Andrews Millikan

The process of introducing members to a new way of viewing the health physics and radiation safety profession brought a wave of ideas to mind.  There are events that took place.  There are people who have explored, discovered, developed and invented materials, methods, equipment and devices which allow us to realize today the advances in technology that have occurred over the years.  We benefit from those who have gone before us.  Just over 100 years ago in 1912 Millikan performed the oil-drop experiment to determine the charge of the electron.  Millikan had a fundamental focus on the importance of precision measurements.  He had a knack and gift for bringing to research pursuits his technical ingenuity and meticulous ability for error analysis.

Robert Andrews Millikan was born on 22 March 1868 in Morrison, Illinois and died of heart failure on 19 December 1953 at home in San Marino, California. Robert Millikan was the most famous American scientist of the 1920s, and the second American (the first American born) to receive the Nobel Prize in physics in 1923 for his study of the elementary electron charge and the photoelectric effect.

Robert was the second son of Reverend Silas Franklin Millikan and Mary Jane Andrews.  Millikan grew up in rural Maquoketa, Iowa and spent time fishing and farming. He excelled in his studies and graduated from Maquoketa High School in 1886.  Waiting for college he worked at a sawmill and as a court reporter.  After his second year at Oberlin College, Millikan was asked to teach the introductory physics course. Millikan earned Bachelor and graduate degrees from Oberlin.  At Oberlin, Millikan discovered his inherited fondness and enthusiasm for teaching and training future generations.  Millikan’s parents had graduated from Oberlin and were educators, his father a minister and his mother dean of women at Olivet College in Michigan. 

In 1893 he began at Columbia University and in 1895 earned the first doctorate in physics ever awarded. Millikan’s doctoral advisors were Michael I. Pupin and Albert A. Michelson. He went to Europe for postgraduate study.  Millikan attended lectures by Henri Poincaré in Paris.  In Germany he studied with Max Planck, Walther Nernst, and Felix Klein.  Millikan credited Nernst for revealing how to combine teaching and research by integrating graduate students into the laboratory.  Today this practice is routine.  In 1896, he returned to Michelson’s lab in Chicago. 

Millikan’s original oil-drop apparatus, circa 1913

Diagram of Millikan's apparatus, from his Physical Review paper.  The air about the drop was ionized when needed by means of Roentgen rays from an X-ray tube which readily passed through the glass window.

Diagram of tube used in photoelectric determination of Planck’s “h.”  Millikan described this “experimental arrangement as a machine shop in vacuo.” Physical Review.

Around 1906-1907 Millikan began one of his most significant research pursuits: determining the charge of an electron with the oil-drop experiment. Millikan set up parallel conducting plates horizontally, with a large adjustable electric field between them (via an X-ray tube). Lit from the side, the drops glistened when the region was viewed through a microscope. With the electric field off, Millikan could observe a falling drop and measure its terminal velocity. The measurement gave him the radius of the drop, and with density known, could determine the mass.  He could then switch on the X-ray tube, and adjust the electric force precisely to balance the force of gravity on the drop. Knowing the strength of the field and the mass of the drop, he could calculate the charge on the drop. Working with Fletcher, Millikan showed the charge of the droplets were always a whole number multiple of 1.592 x 10^-19 C, the basic unit of charge [e = 4.774 x 10^-10 ±0.0095 esu].  Today the accepted value is 1.602 x 10^-19 C. The difference has been attributed to an incorrect value for the viscosity of air (not for any experimental error as some had thought).  Millikan and Fletcher used the oil-drop experiment to measure the charge of the electron (as well as the electron mass, and Avogadro's number, since their relation to the electron charge was known). Millikan determined the Avogadro constant to be N = 6.062 ± 0.012 x 10²³. Today the accepted value is 6.022 x 10²³. On 2 June 1913, he published the results in Physical Review, “On the Elementary Electrical Charge and the Avogadro Constant.”  

From 1912-1915 Millikan made the first direct photoelectric determination of Planck's constant h. Millikan set out to disprove Einstein’s photoelectric effect theory and resulted in confirming Einstein’s predictions were correct. Millikan measured the value of Planck's constant by using photoelectric emission graphs of various metals.

In 1917 Millikan became president of the American Physical Society and in February 1917 moved to Washington, DC from Chicago and was appointed as Executive Officer and Director of Research at the National Research Council. Millikan remained in this position until 1945.  During WWI Millikan served in the Science and Research Division of the Army Signal Corps.

In 1921 Millikan moved to California to lead the California Institute of Technology (Caltech).  At Caltech Millikan studied cosmic rays (a term he coined).  With Ira S. Bowen he advanced the field of vacuum ultraviolet spectroscopy.

Millikan had successful and noteworthy doctoral physics students at Caltech.

• Chung-Yao Chao studied the scattering of gamma rays in lead by pair production.
• Robley Dunglison Evans (also a health physicist) built at MIT the first whole body counter to measure radium uptake by radium dial painters and performed the first quantitative in-vivo radionuclide measurements.
• Harvey Fletcher studied at the University of Chicago, and worked on the oil drop experiment.  Fletcher was known as the "father of stereophonic sound" and invented the audiometer and hearing aid.
• Charles Christian Lauritsen with Ralph D. Bennett developed high voltage X-ray tubes for radiation therapy in the Kellogg Radiation Laboratory.  In 1932 Lauritsen converted an X-ray tube into an accelerator of protons and helium ions to study nuclear reactions.  In 1934 Lauritsen and H. Richard Crane used deuterium (from G.N. Lewis at Berkeley) to generate neutrons and made the first accelerator produced artificial radioactivity.  He later measured the radiation produced when a positron and an electron annihilate each other.  He discovered the radiative capture process.  In 1937 he invented the Lauritsen electroscope, commonly known as the pocket ionization chamber (PIC).
• William Pickering, a New Zealand born rocket scientist headed the Jet Propulsion Laboratory for 22 years.
• Ralph A. Sawyer (also an academic administrator) developed ultraviolet spectroscopy for studies of atomic structure and developed industrial applications of spectroscopy.  In 1946, Sawyer became civilian director of the atomic bomb tests at Bikini Atoll.

 This author hopes you enjoyed this look back at a fundamental figure in today’s world of modern physics and someone that influenced some well-known characters in the history of our profession.